1. Field of the Invention
The present invention relates to variablefocus objective lenses which are more particularly intended to be employed in television studios.
2. Description of the Prior Art
Devices of this type are made up of several groups of lens elements, the displacements of which make it possible to obtain a variation of field at the same time as a still picture on the receiver irrespective of the focusing distance chosen. As a general rule, the function of variation of focal lengths is performed by two movable groups of lenses constituting a so-called "variator" whilst the other groups undergo suitable displacements in order to adjust focusing under the best possible conditions of picture quality up to minimal distances, that is, practically against the front element. In order to obtain the still picture, the laws of displacements of the movable groups are computed and applied in real time as a function of the desired focal length and distance.
In the majority of objective-lens systems employed in television studios, a system is provided in addition for carrying out multiplication of the basic focal length. This system consists of a rotary lens turret placed behind the iris and adapted to carry a number of quasi-afocal lens elements, each of which may thus be put into service in order to obtain a different multiplier coefficient.
This makes it possible to obtain a constant variation ratio and to choose the basic focal length as a function of the field angles which can be employed. However, since the maximum available f-number is proportional to the basic focal length, this must be taken into account in the choice of the multiplier. In fact, by choosing a situation which is conducive to long focal lengths, it may happen that the lens aperture is stopped-down too far. The ideal arrangement, in the event that the quantity of light is critical and that it is therefore desired to give priority to the lens aperture, would be to have a large number of multipliers and to be in a position to choose the best compromise. This is not possible, however, with current systems consisting of a lens turret which carries a limited number of quasi-afocal lens elements corresponding to as many different multiplier coefficients.
Another characteristic of existing objective-lens systems of the type under consideration is that the multiplier system thus provided is located in a quasi-afocal region behind the iris. This actually has the effect of significantly changing the position of the exit pupil and therefore the distribution of colors in the picture (colored spot) as a result of the changes in angle of incidence of the principal beams on the dichroic mirrors of the camera separator. This phenomenon is aggravated when the exit pupil is relatively close to the image and also when "ramping" occurs or in other words when the f-number varies with the focal length. In this case, the exit pupil is in fact the image of the diameter of any one group of the variator and often of the front element, the dimensions of which are always limited so as to reduce the weight and volume, and is no longer the image of the iris. Furthermore, the spectrum transmission is modified by introduction of additional lenses.
It is for the reasons given above that the object of the invention is to permit the application of multiplier coefficients for multiplication of the basic focal length of an objective-lens system of the type considered while removing the disadvantages of the system discussed in the foregoing.